1. Field of the Invention
This invention relates generally to the suppression of interference among communication signals. More particularly, the invention relates to the implementation of statistical techniques to interference suppression.
2. Description of the Related Art
Radios may receive three forms of signals: noise, interference and communication. Noise, created in the atmosphere due to natural causes such as lightning and the like, is unpredictable. Few steps are taken toward its elimination. Interference signals can be caused inadvertently, such as by several stations broadcasting on the same band, or can be caused deliberately, such as by an adversary transmitting signals to mask communication signals. Many techniques have been devised to combat interference.
As major sources of radio interference are non-Gaussian in structure, receivers effective in detecting communication signals in the presence of non-Gaussian interference have been known to estimate the statistics of this interference. This estimate is then used to transform received radio signals into perceptible communication signals.
Adaptive filters have been used to perform the above functions. One adaptive filtering technique creates a transform based upon the derivative of the natural log of the probability density function (PDF) of an interference magnitude, such as signal amplitude. For sampled data, an estimation of the PDF has traditionally been obtained through estimation of a cumulative probability function of the data. Derivatives have been calculated using finite difference operations.
Such adaptive filtering techniques can be computationally complex, incapable of performing in real-time, and numerically unstable. To perform satisfactorily, these techniques often require a relatively great number of signal samples. This high sample requirement makes some adaptive approaches computationally demanding and confines their use to dedicated microprocessors, and then primarily those used with radios operating in the lower portion of the radio frequency spectrum (low frequency [LF], very low frequency [VLF], and extra low frequency [ELF]).
Though successful interference suppression has been obtained through the use of adaptive filters, such filtering devices use a great many calculations and require considerable processing. Their complex design has led to interference processors of high cost.
There is thus a need for an interference suppression system that can be used with dedicated or nondedicated microprocessors, that is suitable for use with radio frequency receivers operating over both the lower and upper portions of the radio frequency spectrum and that is relatively simple in operation, permitting interference suppression at a reasonable cost.